Microwave antenna system



Aug. 1, 1961 o. c. WOODYARD MICROWAVE ANTENNA SYSTEM 2 Sheets-Sheet 1Filed May 23, 1950 PHASE a AMPLITUDE CONTROL HYBRID CIRCUIT W RADARSYSTEM x -x o FIG.3

INVENTOR. ORVILLE C. WOODYARD Aug. 1, 1961 MICROWAVE ANTENNA SYSTEMFiled May 25, 1950 2 Sheets-Sheet 2 INENN: FTEUEI'GR? 27 I 9 2'.

I I4 20 P28 I9 ll l TRANSMITT DUPLEXER l t l-- l l8 I0 I I so 32 35 asTIMER DETECTOR 090. PH. DETECTOR LP. LE 39 AMPLIFIER AMPLIFIER AMPLITUDEPHASE DETECTOR 35 DETECTOR 4o TRACKING cmcun' FIG. 6

INDICATOR 1 V I/ AMPLITUDE A B c ANGLE F|G.7 o

F|G.8 I3 I MAGNETRON I 42 43 r INVENTOR.

45 44 ORVILLE' c. WOODYARD 32 080. 3'7

mi BY I FIG.9 W 7);. @944 United States Patent 2,994,869 MICROWAVEANTENNA SYSTEM Orville C. Woodyard, Neptune, N.J., assignor to theUnited States of America as represented by the Secretary of the ArmyFiled May 23, 1950, Ser. No. 163,762 7 Claims. (Cl. 343-16) v (Grantedunder Title 35, US. Code (1952), sec. 266) The invention describedherein may be manufactured and used by or for the Government forgovernmental purposes, without the payment of any royalty thereon. VThis invention relates to a microwave antenna system for producing ahighly directive radiation pattern of the type employed in radarsystems. In particular it relates to microwave antenna systems employinga wave guide radiator or wave guide feed in a radiating system.Additionally, the invention relates to a microwave antenna system of thewave guide type for producing a radiation pattern of predetermineddirectivity suitably for use in a monopu-lse direction finding system.

Heretofore, in microwave radar systems where it is desired to provide ahighly directive radiation pattern, wave guide radiators or more exactlyantenna systems employing Wave guide radiating feed elements have beenemployed. For example, a familiar radar antenna structure is that of awave guide feed directed to illuminate a parabolic reflector, thestructure being arranged for orientation in azimuth and in elevation todirect a major lobe of radiation in a particular direction. Themicrowave energy is propagated from the mouth of a wave guide located ata focal point of the reflector. Within the guide this radiation isgenerally propagated in the fundamental mode and produces a radiationpattern of fixed directivity.

Inradar systems employing lobe switching, the wave guide feed is joinedto the system so that the mouth of the guide, as directed toward thereflector, may be physically changed in position to alter the directionof the major lobe of radiation. Generally this physical displacement isaccomplished mechanically at a high rate to cause a corresponding shiftin the direction of the radiation at this rate. In other radar systemsof the monopulse or simultaneous lobing type, to which the presentinvention is directed, the practice has been to employ two or moreseparate wave guide feeds directing the microwave energy at a reflectoror lens. The circuit arrangements provide for simultaneously using theplurality of guides to determine the direction of an incoming signal. Todetermine the direction in one coordinate, say azimuth, at least twowave guide feed elements are used. To determine the direction in bothazimuth or elevation, four or more wave guide feed. elements areemployed.

The present invention contemplates in one of its embodiments theemployment of a single wave guide feed instead of the multiplicity ofguides or a single guide that is physically changed in position.

It is accordingly an object of the present invention to provide amicrowave antenna system which avoids one or more of the disadvantagesand limitations of prior art systems.

It is a further object of the present invention to provide a microwaveantenna system comprising a duo-mode wave guide having a radiating openend to produce a radiation pattern having a major lobe of predetermineddirectivity.

It is an additional object of the present invention to provide in aradar system a microwave antenna system comprising a duo-mode wave guidehaving a radiating open end for determining the direction of arrival ofthe reflected wave energy.

In accordance with the'present invention there is pro- Patented Aug. 1,1961 vided a microwave antenna system comprising a duomode wave guidehaving a radiating open end. The guide has dimensions which are chosento propagate wave energy simultaneously in its fundamental and secondorder modes, for example, a rectangular wave guide having dimensionschosen to propagate wave energy in the TE and TE modes. Means are alsoprovided for proportioning the relative amplitude and phase of waveenergy propagated in the two modes to produce a radiation pattern havinga major lobe of predetermined directivity.

Also in accordance with the present invention there is provided in aradar system an antenna structure comprising a duo-mode wave guideradiator. The guide has dimensions chosen to propagate wave energysimultaneously in its fundamental mode and its second order mode, forexample, a rectangular wave guide having dimensions chosen to translatewave energy simultaneously in the TE and TE modes. Means are providedfor utilizing the guide to radiate the wave energy in the fundamentalmode. Further means are provided for utilizing the guide to receive areflected part of the energy in both of the modes and additional meansare provided for combining and detecting the received energy and forutilizing the detected energy to indicate the direction of arrival ofthe reflected energy.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawing and itsscope will be pointed-out in the appended claims.

Referring to the drawings, FIG. 1 is a drawing, partly in block diagramand partly schematic, illustrating. a fundamental embodiment of thepresent invention; FIG. 2 is a diagram corresponding to FIG. 1illustrating the fundamental and second order modes propagated in theguide and the directive radiation pattern which obtains; FIG. 3A and Bare respectively graphs and corresponding vector diagrams illustratingthe wave energy distribution in amplitude and in phase at the mouth ofthe wave guide of FIG. 1 for the purpose of explaining the operation;FIG. 4 is a diagram illustrating a modified form of the invention; FIG.5 illustrates a preferred form of wave guide radiator for use in thepresent invention; FIG. 6 is a block diagram illustrating a monopulse orsimultaneous lobing radar system in accordance with a preferredarrangement of the present invention; FIGS. 7 and 8 are diagramsillustrating the operation of the system of FIG. 6; and FIG. 9 is adiagram illustrating the embodiment of a part of the FIG. 6 arrangementin wave guide form.

Considering now FIG. 1 of the drawing, the arrangement illustrated isthat of a microwave antenna system comprising a rectangular duo-modewave guide 10 having an open end 11; the end or mouth of the guide beingdirected toward a reflector or lens system 12. A radar system isindicated by unit 13 and ordinarily comprises a multiplicity ofelectronic units and elements for transmitting pulses of microwaveenergy and for receiving reflected echo pulses. Unit 13 is coupled towave guide radiator 10 via a first coaxial line or path, 14, fortranslating the fundamental mode and via second path comprising acoaxial path 15, a phase and amplitude control unit '16 and a coaxialpath 17 for translating the second order mode. The coaxial path 14 iscoupled to guide 10 by means of a probe element 18, while the coaxialpath 17 is coupled to guide 10 by means of the probes 19 and 20extending into the guide. The length of the coaxial path between :19 and20 is a half wave length so that the excitation of wave energy producedby these probes is of opposite polarity. This arrangement isconventionalfor introducing or deriving the second order TE mode and isshown for convenience but it will be understood that any of the knownequivalents may be employed.

Before considering the operation of the system certain simple relationsaffecting the propagation in a rectangular guide should be noted, viz:

(a) The cut-elf wave length A i related to the width a and height I) ofthe guide by the equation:

AFFFY where m and n are mode index numbers. From this relation itfollows that to pass the TB mode the width a must be greater than A/ 2,where is the free space wave length. If the guide is also to pass the TEmode the Width a must be greater than )t.

(b) The phase velocity of wave energy in a guide is greater than thefree space velocity and differs for each mode. The wavelength in theguide is correspondingly increased and for the TE and TE modes isdefined respectively by the relations:

a As an example of a practical case, the free space wave length of themicrowave energy is 3.2 cm. and the guide dimensions are chosen with a=4cm. and b=l cm. in order to propagate both the fundamental and secondorder TE modes. For these dimensions the guide wave lengths are hgl=3.5cm. and Ag2=5.33 cm.

Consider now the operation of the system of FIG. 1 with reference toFIGS. 2 and 3 of the drawings. In FIG. 2 the guide 10 is againillustrated showing the positions of probes 18, 19 and 20 in view A.View B is a standard .type of illustration of the potential or electricfield distribution of mircrowave energy at the mouth of the guide forthe fundamental TE mode wherein the potential distribution is indicatedby vertical vectors which are concentrated at the center. Referring toFIG. 3, graph A, this potential distribution across the mouth of theguide is indicated by curve M which shows that the electric intensity ismaximum at the center of the guide and diminishes sinusoidally to zeroat the edges of the guide.

For the second order mode view C of FIG. 2 shows the TE distribution ofthe electric potential across the mouth of the guide and again referringto graph A of FIG. 3 this distribution is illustrated for two differentvalues of intensity, by curve NN' for a low intensity of the secondorder mode and by curve OO for a greater intensity of second order mode.It will be noted that the portion of the curve N is shown in solid lineto indicate, say, positive polarity of the potential relative to theportion N shown in dotted line to indicate negative polarity. SimilarlyO and 0' indicate positive and negative intensities.

The field distribution across the mouth of the guide at distances, x,from the center, 0, may be written as:

e =E cos ITJC/ a cos wt for the TE mode (4) and as:

e =KE sin 21rx/a cos (w.+) for the TE mode (5) where a=width of theguide E=rnaximurn field intensity K=constant w=27rf=angular frequency=phase difference Referring again to the wave guide of FIG. 1, it will 72,994,869 n 7 W V be noted that the location of the probe 18 forpropagating the fundamental mode TB in the rectangular guide is in thecenter line of the guide and will preferably be located a distance fromthe closed rear wall of the guide by a distance labeled L3. Thisdistance L3 will generally be chosen to be a quarter wavelength, Agl/4,in order to help match impedances of the guide and the coaxial line 14.

Similarly it will be noted that the probes l9 and 20 for propagating thesecond order mode are located at positions which are each substantiallyhalf way between the center line of the guide and a side wall. Theseprobes are located a distance L4 from the rear wall of the guide whichis also substantially a quarter wavelength, )tg2/4.

It will be clear that, since lgl is less than l\g2, the location ofprobe 18 is closer to the rear wall of the guide than the locations ofprobes 19 and 2b. The distances of the probes from the mouth 11 labeledL and L are also dif ferent and it will be evident that the waves willarrive at the mouth of the guide with a phase difference as in dicatedin the equations. However, unit 16 may be adjusted to control theamplitude and phase of the second order mode and in accordance with theinvention the phase adjustment will be made such that the phasedifference measured at the mouth of the guide is ninety electricaldegrees. The amplitude, as will presently be explained, will be chosento produce a predetermined directivity of the major lobe as measured ina plane normal to the electric vector.

The radiation pattern is shown in FIG. 2 as a single major lobe with twosmall minor lobes and is a somewhat idealized illustration but one whichsubstantially shows the operation. Shown in solid line the lobe 21 isnormal to the mouth of the guide and represents the radiation patternthat would be produced by exciting the guide with only the fundamentalmode for transmission or utilizing only the fundamental mode inreception. In other words the directivity is along the center line oraxis of the guide when only the fundamental mode is employed. For thiscondition the phase of wave energy at all points across the mouth 11 isthe same. This condition is indicated by the vertical vectors in thediagram B of FIG. 3. Here points x x etc., equally spaced across theguide mouth, have been chosen to picfront in the plane of the mouth 11to propagate the major part of the energy normal thereto. It is alsowell known that if the amplitude is tapered so that the radiation is amaximum at the center and diminishes gradually to zero at the edges thenthe minor lobes of the radiation pattern are reduced although the majordirective lobe may be somewhat broadened.

If the phase distribution varies linearly across the mouth 11 the wavefront will be tilted to direct the ma or lobe at any angle 0. Forradiation directed at an angle 0, a good distribution of amplitude andphase at the mouth 11 would be one of sinusoidal amplitude to reduceminor lobes and of linear phase to determine the directivity. Thisdistribution can be defined by the equation:

11-2: :04 e E'eos a cos (wt+; (6)

Where I is the total phase shift from the center 0 to an edge of theguide.

The wave energy received at a distant point in a direction of angle 0from an element of aperture may be written as and a maximum of radiationwill be received at that point when, for all elements,

:0 2am a/-2-+ T (308 0- 0 Accordingly for linear phase distribution atotal phase shift of is the amount required to tilt the major lobe inthe direction 0. Such a tilted lobe 21. is shown in dotted line in FIG.2.

The described amplitude-phase distribution can be approached closely forsmall angles a of tilt from the axis of the guide (e.g., large angles 0)by introducing the second order mode. Consider the potentialdistribution across the mouth of the wave guide due to the fundamentalmode to be as indicated by curve M in FIG. 3 and that a certain amountof second order mode is supplied to the guide by adjustment of theamplitude in unit 16 of FIG. 1 and that this amount is represented bythe curve N, N. If we further consider that an adjustment is made inunit 16 to control the phase of the second order mode energy supplied toprobes 19, 20 so that the wave, when propagated in the guide, will reachthe mouth of the guide 11 in phase quadrature with that energy suppliedand propagated in fundamental mode, then the wave energy at the mouth ofthe guide is the vector sum of the fundamental and second order modes.

That is, the phase of the second order mode is in time quadrature andhence corresponding points on curves M and N, N must be added inquadrature. 'This is shown by the vectors for positions x x and x in Bof FIG. 3. The component of N for position x is therefore the smallvector 50 indicated as lagging 90 from the reference vector and theaddition produces a resultant vector labeled to indicate that theresultant vector lags the reference vector by that amount. For theposition x the component of curve N is a larger vector, as illustrated,lagging 90 and its. addition to the reference vector is indicated as aresultant vector labeled 20 to indicate that the resultant lag is 20from the reference vector. For position x the component of curve N to beadded in quadrature is smaller but the reference vector to which it isadded is also smaller so that the resultant vector is as labeled, 28.The component and resultant vectors have not been drawn for the lefthand side of the diagram but it will be evident that since thesecomponents of curve N are of opposite polarity to those of curve N theresultant vectors for negative distances x from the center of the mouthof the guide will lead the reference vectors. In other words, theresultant vectors for points x on the left side of the guide will beconjugate to those on the right side. If similar vectors were drawn forall positions x along the guide, it will be evident that the angle oflead or lag relative to the center 0 is substantially linear. Theexample has illustrated that for the distance x the lag is 10, for thedistance x which is twice x the lag of 20 and for the position of x;;which is three times the distance x the lag is 28 which is nearly thatof the ideal of- 30". In other words the injection of second order modecomponents corresponding in amplitude to the potential distribution N,N' and arriving at the mouth of the guide in phase quadrature to thewave energy of fundamental mode produces substantially a linear phasedistribution of wave energy across the mouth of the wave guide and wherenear the side walls of the guide this phase distribution begins tofail,-the amplitude of the energy is so relatively small that thefailure of linearity 6 of phase in this region is negligible. Ittherefore follows from graphical description that injecting a smallcomponent of second order mode will produce a tilt of the major lobe ofthe directional pattern as indicated for example by the lobe 21' of FIG.2.

If now we consider the injection of wave energy of the second order modein larger amplitude as shown by the curve 0, O of FIG. 3 and considerthat the adjustment of 16 has been so made that again the wave energyarrives at the mouth of the guide in phase quadrature to the energyarriving in the fundamental mode, then the vector addition asillustrated in B of FIG. 3 indicates that for positions x x and x thephase lag is respectively 20, 36, and 44 which is still not too far fromthe ideal phasing of 20, 40 and For the posit-ions x x x similar butleading conjugate vectors will be produced. It will be evident, however,that the departure from the linear phase distribution across the mouthof the guide is greater for larger injections of the second order modeso that while the major lobe will be turned or tilted through a largerangle we may expect that the lobe will be somewhat distorted and thatminor lobes of greater intensity may appear. It follows, therefore, thatdirectional control of the radiated major lobe pattern can beaccomplished by employing components of the fundamental and second ordermodes for angles a that do not depart too far from the axis of theguide. For most purposes, particularly in a radar system for angle orrange tracking where use of the antenna system of the present inventionis contemplated, the degree of control it provides is entirely adequate.

The description of operation has been chiefly from the transmittingpoint of view in showing that the introduc: tion of a second order modecomponent will produce a substantially linear phase characteristic atthe mouth or open radiating end of the guide. However, as in all antennaradiating systems, the reciprocity theorem operates so that an incomingwave incident at the open end 11 will be received with similardirectivity.

Thus, for reciprocal receiving operation with the same directionalpattern, in the FIG. 1 arrangement the unit 16 serves to proportion theenergy translated in second order mode to unit 13 and to adjust thephase.- In practice no actual unit 16 need be employed but instead asingle coaxial path from 13 to the probes 19 and 20 might be employedhaving a length relative to the path 14 chosen to produce the desiredrelative phase and having a characteristic impedance chosen toproportion the amplitudes of the modes. Suitable matching sections willof course be employed where needed in a manner well known in the art.

Referring now to FIG. 4 there is illustrated in block diagram analternative arrangement of the present invention which employs only twoprobes which are 10 cated in the guide 10 at positions 19 and 20'. Thesepositions are spaced from the walls of the guide as in FIG. 1 for thepositions of probes 19 and 20 but are to' be here employed to establishboth fundamental and second order modes. Therefore their locationrelative to the closed end of the guide is roughly an average quarterwavelength; that is the distances L will be an average of Agl/4 and AgZ/4. The coupling to the probes 19' and 20' is via the two paths 14 and 15corresponding to the similarly labeled paths of FIG. 1 and thencethrough a hybrid circuit 22. Hybrid circuits are now well known in theart and generally for microwave operation will be in the form of a ringguide circuit or a magic-T guide circuit of the types shown, forexample, in US. Patent to' W. A. Tyrrell No. 2,445,895 issued July 27,1948. The hybrid circuit 22 here indicated by block diagram ischaracterized by having two balanced positions 23 and 24 where the paths14 and 15 connect and two side positions 25 and 26 where connections aremade to the probe locations 19' and 20 respectively. Considering theoperation from the transmitting point of view, energy supplied viaconnection 14 to position 23'will be translated from side positions 25,26 to the probe locations 19' and 20' in time phase but will not betranslated to position 24 and back into line since 23 and 24 arebalanced or conjugate positions. Similarly wave energy supplied viaconnection 15 to position 24 will be translated from side positions 25and 26 in opposite phase to probe locations 19' and but will not betranslated to the balanced position 23 and back into line 14.

Because of reciprocity of operation discussed above, it will be evidentthat, in receiving, a wave incident at the mouth of the guide willproduce a component of fundamental mode which will be propagated downthe guide to elements 19' and 20 and a component of second order modewhich will be propagated at a different velocity down the guide to thesesame elements, the relative amplitude of these components correspondingto the direction of arrival of the incident wave. The wave offundamental mode travelling in the guide, is of equal intensity at theseprobe locations and in time phase and therefore passes to the hybridcircuit at positions and 26 and leaves the circuit at position 23 viapath 14 but does not produce any output at the balanced position 24. Thesecond order mode, however, travelling down the guide provides equalamplitudes but opposite phase of wave energy at the probe positions 19'and 20' and this energy which also enters the hybrid circuit atpositions 25 and 26, being of opposite polarity or phase at these twopositions, leaves the hybrid circuit at position 24 by con nection 15but does not produce any output at position 23. The arrangementtherefore operates in a manner entirely similar to that described forthe arrangement of FIG. 1 in propagating both fundamental and secondorder modes of wave energy in the guide 10 but some compromise has beenintroduced to reduce the arrangement to two probes. Referring now toFIG. 5 an improved arrangement is shown for independently translatingwave energy of the fundamental and second order modes to and from arectangular duo-mode wave guide. The arrangement insofar as it relatesto efliciently and independently coupling between wave guides of singlemode and a duomode guide is not a part of the present invention. Its usein a duo-mode guide antenna system in accordance with the presentinvention is, however, preferable to that of the FIGS. 1 and 4arrangements since it provides a better matching of impedences for thetwo paths between the wave energy apparatus 13 and the guide 10. Herethe standard section of single mode wave guide 14' corresponds to thefeed 14 of FIG. 1, the standard section of wave guide 17 corresponds tothe coaxial feed line 17 of FIG. 1 and the section 10 corresponds to theduomode guide radiator 10 of FIG. 1. It will be evident that thearrangement is a form of magic-T hybrid circuit for coupling to aduo-mode guide. and 17' as joined to the duo-mode section 10 correspondto the hybrid circuit 22 of FIG. 4.

Referring now to FIG. 6 there is illustrated a preferred embodiment ofthe invention in a radar system wherein the antenna structure comprisesa rectangular duo-mode wave guide radiator having dimensions chosen totranslate Wave energy simultaneously in the fundamental TE mode and inthe second order TE mode together with means for utilizing the guide toradiate the Wave energy in the fundamental mode. The arrangementincludes means for utilizing the guide to receive a reflected part ofthe energy in both of the modes together wtih means for combining anddetecting the received energy and utilizing the detected energy toindicate the direction of arrival of the reflected energy. In thediagram elements corresponding to those illustrated in FIG. 1 aresimilarly labeled. Thus 27 is an antenna structure which includes theduo-mode guide 10 having a radiating open end 11 directed toward thereflector 12. The guide is equipped with a probe 18 as in FIG. 1 for Theguide sections 14 coupling the fundamental mode and probes 19 and 20connected by a half Wavelength path as in FIG. 1 for coupling the secondorder mode. The antenna structure 27 is arranged to be oriented inazimuth and in elevation by means of a control mechanism indicatedgenerally by block unit 28 and a control connection 29. Unit 28 may bean arrangement partly mechanical and partly electrical such as a servocontrol link and therefore has been indicated in dotted line. Thetransmitter 13 is here assumed to be a radar microwave pulse transmitterwhich is controlled via a timer unit 31} to transmit narrow pulses ofmicrowave energy at a chosen repetition rate determined by the timer.The wave energy output of 13 is coupled to duplexer 31, one output ofwhich is coupled via path 14 to probe 18. Another output of 31 iscoupled to the detector 32 of a receiving channel. Duplexer 31 may be ofany well known form, its function being to translate the wave energyfrom 13 to probe 18 while preventing the energy from entering detector32. Conversely unit 31 operates to translate received echo signals fromprobe 18 to detector 32 and prevents the received energy from enteringtransmitter 13. Wave energy from oscillator 33 is also coupled todetector 32. The output of detector 32 is coupled to IF. amplifier 34and one output of 34 is coupled to an amplitude detector 35. The outputof 35 is coupled to an indicator 36 for indicating the reflected wave.Indicator 36 may be, for example, an oscilloscope for indicating range.A second channel connecting to probes 19 and 20 for receiving the secondorder mode is comprised of detector 37 to which an output of oscillator33 is coupled via a phase control unit 38. The output of detector 37 iscoupled to LP. amplifier 39 which in turn is coupled to phase detector40. An output of LP. amplifier 34 is also coupled to phase detector 40.The output of phase detector 40 is coupled to a tracking circuit unit41. The control potential output of tracking circuit 41 is supplied bythe connection 29 to control unit 28 to orient the antenna structure 27to track the received echo signal.

The unit 41 may also comprise range circuits for tracking the chosentarget in range and to exclude echo signals from targets of other range.For this purpose synchronous operation with the pulse repetition rate isprovided by the coupling connections from timer 39. The output from unit41 to indicator 36 provides a suitable time base synchronous with thepulsing rate for displaying the signal output of detector 35.

Considering now the operation of the system, pulses of microwave energyfrom transmitter 13 enter duplexer 31 where the energy is prevented fromreaching detector 32 and is directly translated via path 14 to probe 18to energize the wave guide radiator 10. The transmitted energy istherefore established within guide 10 entirely in fundamental mode andso travels to the mouth 11 of the guide to be radiated via reflector 12.The energy propagated in the guide is balanced with respect to probes 19and 20 which select and translate only wave energy of second order modeand therefore cannot reach the input of detector 37. The arrangementthus far described comprises means for utilizing the guide 10 to radiateenergy in the fundamental mode.

Consider now the operation of the system in receiving when wave energyfrom a particular reflecting object is received by the antenna structureand enters the mouth 11 of guide 10. This wave energy, if it arrivesfrom a direction which is not normal to the mouth of the guide, producescomponents of fundamental mode and of second order mode. The componentof fundamental mode is picked up by the probe at location 18 andtranslated via connection 14 to duplexer 31 where it is prevented fromreaching transmitter 13 but is translated to detecor 32. In the detector32, to which wave energy from oscillator 33 is also coupled, thereceived signal is converted to intermediate frequency which isamplified by unit 34. The LP. energy from 34 is supplied to amplitudedetec- 9. tor-35 where it is detected to provide a video signal. Thevideo signal output 35 is supplied to indicator 36 to indicate thisreceived signal.

The component of second order. mode is picked up by the probes 19 and 20and conveyed via the connection 17 to the input of detector 37. Waveenergy from beating oscillator 33 is also supplied to detector 37 inphase determined by the adjustment of unit 38. The output of 37 istherefore wave energy of the same intermediate frequency as thattranslated by I.F. amplifier 34 but of predetermined phase as determinedby the setting of phase control unit 38. The LP. energy output from unit34 and LP. energy output of unit 39 are both supplied to the phasedetector 40. Phase detectors of a variety of forms are known in the artso that no particular form of phase detector need be described here. Thebook Waveforms, vol. 19 of the Radiation Laboratories Series-McGnaw-HillBook 00., pages 511-513 illustrates and described a number of suitablephase detector circuits.

The operation of the phase detector may be visualized by reference tothe vector diagrams of FIG. 7. The LF.

wave energy from unit 34 constitutes a voltage of reference. phase andis so indicated in FIG. 7 by the vertical vector at A; The wave energyoutput of intermediate frequency from I.F. amplifier 39 is set by phasecontrol unit 38 to be in quadnature phase. This is illustrated at B,ofFIG. 7 where a resultant vector is shown which is the sum of thereference vertical component from unit 34 and the horizontal quadraturecomponent from unit 39. The phase of the resultant vector relative tothe reference vector is measured by the phase detector 40 which producesa voltage of amplitude and polarity corresponding to the magnitude andsign of the phase angle. If the incoming wave energy is arriving from anangle which is further from the direction line of the antenna thequadrature component will be greater and the vector diagram C of FIG. 7illustrates this condition where the resultant vector has a greaterphase angle. If the incoming wave is from an opposite direction thequadrature component will be reversed in sign and the resultant vectorwill have an opposite phase angle. It will be evident therefore that thephase detector 40 has a characteristic such as that shown in FIG. 8.FIG. 8 indicates that when the wave energy is arriving in the directionline of the antenna the output of phase detector 40 is zero as shown atO in the diagram but that for negative angles, a, of arrival the signaloutput is a voltage of positive polarity and for positive angles ofarrival the voltage is of negative polarity. It will be clear thereforethat this characteristic may be employed to provide angle tracking in amanner well known in the 'art. Thus if the incoming signal is arrivingfrom a positive angle a from the direction line of the antenna, negativevoltage will be supplied via the connection 29 to control unit 28 whichthen operates to turn the antenna in a direction to make this voltagezero. If the incoming signal is from a negative angle a the voltagesupplied by connection 29 will be positive to turn the antenna structurein the opposite direction so that the antenna structure willautomatically seek and center itself so that its direction line willcoincide with the direction of arrival of the incoming signal.

FIG. 9 is a redrawing of the microwave portion of FIG. 6 to illustratein schematic form the microwave guide assembly. The duo-mode radiator10' and connecting guides 14 and 17 are of the preferred form shown inFIG. 5. The transmitter 13 is labeled to indicate the use of magnetronand the duplexer 31 is here comprised of the anti-transmit-receive(A.T.R.) unit 31 and the transmit-receive (T-R) unit 31" commonlyemployed in microwave guide apparatus. The detectors 32 and 37 areindicated as crystal detectors located respectively in resonant cavities42 and 44. Oscillator 33 which may be of the klystron type is coupled tocavity 42 via a coupling loop 43 and to cavity 44 via a coupling 10 loop45. The line 46 connecting oscillator 33 to cavity 44 may be of a chosenlength to provide suitable phasing and so becomes the equivalent of thephase adjusting unit 38 of FIG. 6. I

The arrangements shown and described above disclose a directive antennasystem for controlling and utilizing directivity in one dimension, say,the horizontal or the vertical. For controlling and utilizingdirectivity in two dimensions a pair of duo-mode guides may be employedor a single guide can be chosen and arranged for operation with thefundamental and two other second order modes in accordance with theprinciples of the invention as described for one dimension.

The invention has been illustrated for operation with the TB and T13modes in a rectangular guide but it will be clear to those skilled inthe art that other related modes may be used and other than rectangularguides may also be employed. For example it will be evident that a roundwave guide operating in the TE and TE modes may be so employed.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is:

1. A microwave antenna system comprising a duomode wave guide having aradiating open end, said guide having dimensions chosen to propagatewave energy simultaneously in the fundamental and second order modes andmeans operatively associated with said guide for proportioning therelative amplitude and phase of wave energy propagated in said two modesto produce a radiation pattern having a major lobe of predetermineddirectivity.

2. A microwave antenna system comprising a rectangular duo-mode waveguide having a radiating open end, said guide having dimensions chosento propagate wave energy simultaneously in the TE and TE modes and meansoperatively associated with said guide for proportioning the relativeamplitude and phase of wave energy propagated in said two modes toproduce a radiation pattern having a major lobe of predetermineddirectivity.

3. A microwave antenna system for producing a radiation pattern having amajor lobe of predetermined directivity comprising a duo-mode wave guidehaving a radiating open end, said guide having dimensions chosen topropagate wave energy simultaneously in the fundamental and second ordermode, means for separately translating the wave energy of said modes toor from said guide over two different paths and means for adjusting therelative amplitude and phase of the wave energy translated over saidpaths.

4. A microwave antenna system for producing a radiation pattern having amajor lobe of predetermined directivity comprising a rectangularduo-mode wave guide having a radiating open end, said guide havingdimensions chosen to propagate wave energy simultaneously in the TE andTEQU mode, means for separately translating the wave energy of saidmodes to or from said guide over two diiferent paths and means foradjusting the relative amplitude and phase of the wave energy translatedover said paths.

5. A microwave antenna system for use with wave energy apparatus toproduce a radiation pattern having a major lobe of predetermineddirectivity comprising a duo-mode wave guide having a radiating openend, said guide having dimensions chosen to propagate Wave energysimultaneously in the fundamental and second order modes, means couplingbetween said guide and said apparatus comprising means for separatelytranslating the wave energy of said modes between said apparatus andsaid guide over two different paths and means included in one of saidpaths for adjusting the relative amplitude and phase of the wave energytranslated over said paths.

6. A microwave antenna system for use with wave energy apparatus toproduce a radiation pattern having a major lobe of predetermineddirectivity comprising a duo-mode wave guide having a radiating openend, said guide having dimensions chosen to propagate wave energysimultaneously in the TE and TE modes, means coupling between said guideand said apparatus comprising means for separately translating the waveenergy of said modes between said apparatus and said guide over twodifferent paths and means included in one of said paths for adjustingthe relative amplitude and phase of the wave energy translated over saidpaths.

7. In a radar system an antenna structure comprising a duo-mode waveguide radiator, said guide having dimensions chosen to translate waveenergy simultaneously in the fundamental and second order modes, meansfor utilizing said guide to radiate energy in said funda mental mode,means for utilizing said guide to receive a reflected part of saidenergy in both of said modes, means for translating the wave energyreceived in said modes separately over two different paths, means foreffectively adjusting the relative phase of the output energy from saidpaths and means for combining and phase detecting said energy to producea potential indicating the direction of arrival of said reflectedenergy.

8. In a radar system an antenna structure comprising a duo-mode waveguide radiator, said guide having dimensions chosen to translate waveenergy simultaneously in the TE and TE modes, means for utilizing saidguide to radiate energy in said TE mode, means for utilizing said guideto receive a reflected part of said energy in both of said modes, meansfor translating the wave energy received in said modes separately overtwo different paths, means for efiectively adjusting the relative phaseof the output energy from said paths and means for combining and phasedetecting said energy to produce a potential indicating the direction ofarrival of said reflected energy.

9. A microwave antenna system comprising a duomode wave guide radiatorhaving a radiating open end, said guide having dimensions chosen topropagate wave energy simultaneously in the fundamental and the secondorder modes and means operatively associated with said guide forutilizing the relative amplitude and phase of wave energy propagated insaid two modes to control the directivity of the antenna.

10. A microwave antenna system comprising a rectangular duo-mode waveguide radiator having a radiating open end, said guide having dimensionschosen to propagate wave energy simultaneously in the T15 and TE modesand means operatively associated with said guide for utilizing therelative amplitude and phase of wave energy propagated in said two modesto control the directivity of the antenna.

References Cited in the file of this patent UNITED STATES PATENTSMontgomery Mar. 18,

OTHER REFERENCES Reprint from Proc. IRE, vol. 37, No. 9, September 1949,p. 1031.

